An aircraft is in trim when the vector sum of the forces and moments about its center of gravity is zero. The "y" axis of an aircraft extends laterally out of the aircraft through the center of gravity. The moments acting about this axis are termed the "pitching" moments. In general, the wing generated forces tend to rotate the nose of the aircraft downwardly and the horizontal stabilizer generated forces tend to rotate the nose of the aircraft upwardly. Aerodynamic forces acting on the fuselage also provide a pitching moment input. In an aircraft of a type having engine thrust vectors located below the center-of-gravity, the thrust of the engines tends to rotate the nose of the aircraft upwardly. Any change in position of the flaps at the trailing edges of the wings, or the angle of the stabilizer, or the angle of the elevators at the trailing edges of the stabilizer, or the thrust level of the engines, or the position of the center of gravity of the aircraft will make a change in the net pitching moment.
An aircraft is in trim in the longitudinal direction if the vector sum of the moments about the "y" axis of the aircraft is zero. The forces produced by the wing and stabilizer surfaces are a function of airspeed. The pitching moment input of the fuselage is also a function of airspeed. If an aircraft is in trim and then experiences a change in airspeed, caused by a gust, for example, this will result in a change in the aerodynamic forces acting on the aircraft and an unbalancing of the pitching moments, causing the nose of the aircraft to either move up or down.
Suppose the aircraft is in trim and is disturbed by a gust in a manner causing a change in airspeed. If this change in airspeed produces a nose-up or nose-down moment, tending to move the aircraft even further from its trimmed airspeed, the aircraft is deemed to at that time be longitudinally unstable.
Another measure of an airplane's longitudinal stability is termed the phugoid characteristics. An aircraft flying at a nearly constant angle of attack may tend to oscillate a slight amount in the vertical direction. The extent is termed the phugoid characteristic of the aircraft. During flight in the phugoid mode the pitch angle and altitude change in a manner maintaining a nearly constant angle of attack. Beginning at the top of a cycle, the aircraft slows down to its minimum airspeed and its pitch attitude is nose-up. It then begins to lose altitude. As it does so, its airspeed increases, followed by a nose-down attitude. At the bottom of the cycle its airspeed is a maximum and its attitude is nose-down. It then begins to climb. The airspeed begins to drop off, followed by a nose-up attitude, and then the cycle is repeated. If after perturbation these pitch attitude variations tend to diminish with time without pilot input, the aircraft is said to have stable phugoid characteristics.
The Boeing 757 aircraft includes instrumentation which changes stabilizer angle of attack as function of a change in airspeed, for augmenting longitudinal static stability of the aircraft. A problem with that particular system is that in some aircraft it destabilizes the aircraft's phugoid characteristics.
A principal object of the present invention is to provide a unique system and method for augmenting longitudinal stability by inputting an incremental horizontal stabilizer value that is a function of the change in airspeed from trim in a way that benefits both static longitudinal and phugoid stability.